<!DOCTYPE html>
<html lang="en">

<head>
    <style>
        * {
            margin: 0;
            padding: 0;
        }
        div{
            width: 200px;
            height: 200px;
            /* background-color: rgba(0, 0, 0, 0.486); */
            box-sizing: border-box;
            border-top: 100px solid #000;
            border-left: 100px solid #fff;
            border-right: 100px solid #fff;
            border-bottom: 100px solid #fff;
        }
    </style>
</head>

<body>
    <div id="box">
    </div>
</body>
<script>

</script>

</html>